1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, and more particularly to a control apparatus for an internal combustion engine for detecting the abnormality of a knock sensor.
2. Description of the Related Art
In recent years, a so-called knock control system capable of detecting the knock of an internal combustion engine and of controlling an ignition timing is applied to a large number of control apparatuses for internal combustion engines. Also, as a method of judging the abnormality of a knock sensor for detecting the vibrations of an internal combustion engine, there is a method including: biasing a knock sensor with a resistor by pulling up within an interface (hereinafter referred to as “I/F”) circuit of a control apparatus; and detecting a direct current component of the bias voltage, to thereby detect the abnormality of the knock sensor (for example, refer to JP 60-100021 A).
In the above-mentioned conventional control apparatus for an internal combustion, a built-in resistor is connected in parallel with the knock sensor, one end of the resistor is connected to a ground level through a sensor main body, and only the other end of the resistor is connected to the control apparatus through a harness. The I/F circuit of the control apparatus is pulled up to, for example, 5 V through a resistor having the same resistance as that of the built-in resistor of the knock sensor. Then, only an alternating current component is allowed to pass through a signal processing circuit for detecting the knock through a capacitor, and a signal from which the alternating current component has been removed through a low pass filter that is made up of a resistor and a capacitor is outputted to an abnormality judging A/D of the knock sensor.
However, in the conventional control apparatus for an internal combustion engine, for example, an A/D value obtained from the abnormality judging A/D when abnormality occurs in a harness portion becomes the same value, for example, 5 V, at the time of power supply short-circuiting and at the time of breaking. Therefore, it cannot be discriminated whether the abnormality is derived by power supply short-circuiting or breaking. The A/D value may become 5 V or higher (for example, 12 to 14 V of a battery voltage) in the normal power supply short-circuiting. However, in general, the maximum voltage of A/D is frequently limited to 5 V in order to protect a microcomputer. Therefore, it is hereinafter assumed that a value that has been thus limited is the A/D value in the present specification.
Also, the types of the knock sensor include: a type in which one end of the resistor is connected to the ground level whereas one line of only the other end outputs a sensor signal; and a type in which two lines that are isolated from a constant voltage system such as the ground level output sensor signals. In the case of the two line type sensor, when the abnormality is going to be judged in the same method as the conventional method, there is proposed, for example, a method in which one of two lines is connected to the ground level within the I/F circuit of the control apparatus, and the other line conducts the abnormality judgment in the same method as the conventional method. However, this method suffers from problems in that the abnormality cannot be judged in the case where the line connected to the ground level is grounded, and the same value obtained at the time of power supply short-circuiting between those two lines becomes identical with that at the time of grounding the sensor output line. In addition, there arises such a problem in that a noise removal effect that is attributable to an input of the outputs from the two lines to a differential amplifier circuit cannot be obtained.
Also, in the case where it cannot be discriminated whether the abnormality is caused by power supply short-circuiting or breaking, even if, for example, a lamp indicative of the abnormality is lighted and the control apparatus is then brought to a dealer, it may take time to investigate an abnormal portion unless an abnormal mode is discriminated. Also, since the knock sensor is basically out of contact with a battery voltage system, there is a strong possibility that the power supply short-circuiting abnormality is caused by a harness or a connector being in contact with the battery voltage system. In addition, such a power supply short-circuiting abnormality may evolve into a serious accident such as excess current flow if the abnormality occurs although the event probability is low.
In addition, most of the breaking abnormalities occur owing to a failure of contact with the sensor, the harness, or the connector, and the breaking abnormality is relatively high in the event probability. However, there is a weak possibility that the breaking abnormality develops into a serious accident by conducting appropriate fail safe processing such as retardation of an ignition timing at the time of occurrence of the breaking abnormality. Under the circumstances, it may be preferably possible to discriminate between the power supply short-circuiting abnormality that is low in the event probability but may develop into a serious accident and the breaking abnormality that is relatively high in the event probability but can surely conduct the fail safe processing.